Biogas is a methane-rich gaseous mixture which can be produced through anaerobic digestion of a variety of sources such as landfill waste, animal waste, waste water, food waste, and industrial waste. It is known by various names based on its source; for instance, biogas obtained from landfills is termed as landfill gas (LFG); the one obtained from waste water treatment plants (WWTP) is termed as digester gas (DG); and the one obtained from sludge waste digester is called sludge gas (SG). Approximately 55 million metric tons of carbon equivalent are released into the air each year by landfills in USA alone. The main driving force for the utilization of biogas is to avoid greenhouse gas (GHG) emissions and to lower gas emissions with extremely high ozone depletion potential (ODP). Its utilization will not only reduce uncontrolled emissions of GHGs but will also help to eliminate the wide range of pollutants found in this type of gas, which pose a threat to human health. The main constituents of biogas are methane (40-60%) and carbon dioxide (35-50%) [1]. Methane is a 21-25 times more powerful GHG than CO2 [2]. Additionally, biogas contains small amounts of oxygen (0-3%) and nitrogen, and is also saturated with water vapor. Apart from its main components, biogas (specifically, LFG) also contains relatively high amounts of hydrogen sulfide and a broad spectrum of volatile organic compounds (VOC): organic-sulfur compounds (e.g. carbonyl sulphide, mercaptans), silicon-containing compounds (e.g. siloxanes), halogenated compounds, aromatics and aliphatic hydrocarbons [1]. The high energy content of biogas makes it an interesting energy source [3]. Due to its characteristics, treated (cleaned) biogas is already widely utilized in gas engines and turbines to produce heat and electricity [3]. The utilization of biogas, especially LFG, without treatment for power generation results in damage of downstream power generating units, since during combustion the halogenated compounds and sulfur-containing compounds are transformed into acid gases like H2SO4, HCl and HF, which cause corrosion problems. In addition, siloxanes are transformed into micro-crystalline silica, which deposits on the engine parts contributing to abrasion and poorer combustion efficiency [4]. If gas extraction rates do not warrant direct use or electricity generation, the gas can be flared. Less than 100 m3/h is a practical threshold for flaring [5]. In the landfill gas control hierarchy, gas collection with energy recovery is preferred to flaring [6]. Adsorption and absorption are the two most common technologies used in secondary gas treatment processing [6]. However, the state-of-the-art gas treating processes often fail because of technical or economic reasons, for example, low removal efficiencies, or high operational costs [7]. The World Bank's Global Gas Reduction Flaring Partnership (GGFR) is focused on reducing the CO2 emissions arising from flaring of CH4 associated with biogas such as from LFG and other sources [8]. Sustainable use of biogas for energy production does not contribute to CO2 emissions production but has a high CO2 abatement potential [9]. Therefore, there is immense potential in developing technology to utilize as-generated biogas for energy-related applications.
The major technological challenge for CO2 reforming of biogas is the development of contaminant tolerant catalysts since biogas gas contains two main contaminants namely hydrogen sulfide (H2S) and siloxanes. Biogas composition varies considerably depending on the origin and composition of the biomass and also on the method through which it is generated (thermophilic, mesophilic or psychrophilic). Most of the commonly employed reforming catalysts are prone to deactivation in the presence of H2S and other sulfur compounds [12]. The deactivation sets in on account of the formation of surface oxy-sulfides and/or sulfides [13]. Therefore, it poses a huge technological challenge to devise sulfur tolerant catalyst for biogas reforming [14]. To the best of the inventors' knowledge, there are no reports in the literature on the development of sulfur tolerant catalysts for CO2 reforming of biogas.
Carbon formation is the other main drawback of CO2 reforming of biogas, but second in importance to the earlier mentioned sulfur poisoning [13]. CO2 reforming of biogas uses a high C/H feedstock which results in carbon deposition on the catalyst by CO disproportionation (2CO→CO2+C) and/or methane decomposition (CH4→2H2+C) reactions [15]. The catalysts prepared with noble metals such as Rh, Ru and Pt showed the higher activity performance in CO2 reforming of biogas because noble metals are very resistant to carbon formation. However, these materials are very expensive. Analysis shows that base metals such as Ni-based catalysts have high activity similar to that of noble metals, and are inexpensive. However, nickel catalysts are prone to coke formation [15-17]. Based on a thorough literature review, and to the best of the inventors' knowledge, it can be claimed that there are no reports on the development of sulfur tolerant working catalysts for CO2 reforming of biogas in the literature. A recent publication of Mojdeh Ashrafi reports the use of a commercial catalyst ‘Sued-Chemie G-90’ for the steam reforming of biogas [18]. Most of the research papers dealing with biogas reforming have employed a treated/cleaned biogas (H2S and siloxane free) for their catalytic tests [19-21].
State of the Art in CO2 Reforming of CH4 
The Hydrogen Production Research Group (HPRG) at the University of Regina has considerable experience and expertise in the field of CO2 reforming of methane [22, 23]. The HPRG group has reported the development of 5 wt. % Ni/Ce0.6Zr0.4O2 catalyst for the CO2 reforming of CH4 in 2006 and tested the same for 230 h for the above application [23-26]. In their most recent publication, the use of steam to assist in the CO2 reforming (H2O/CO2/CH4) process in an attempt to mitigate coke deposition over 5 wt. % Ni/Ce0.6Zr0.4O2 binary oxide supported catalysts was reported [27]. The results obtained were not so encouraging. The inherent hydrophilic nature of the ceria-zirconia support offered reduced sensitivity to water inhibition of active sites leading to catalyst deactivation [27]. Thus, in order to find a better catalyst formulation with improved surface structure, morphology, reducibility, redox ability, basicity and steam tolerance (i.e. reduced hydrophilicity), a portfolio of ternary oxide supports of the general formula Ce0.5Zr0.33M0.17O2 were synthesized using the surfactant (CTAB) assisted route [28] and their catalytic activity was evaluated for CO2 reforming of CH4 at 800° C. in the presence and absence of steam in comparison with those for binary oxides supported catalysts. The results obtained were very good and three catalysts formulation namely 5Ni/Ce0.5Zr0.33Mo0.17O2 (where M=Ca, La, Y) were found to be active for CO2 reforming of CH4 both in the presence and absence of steam. Furthermore, their ability to catalyze CO2 reforming of CH4 in the presence of steam was established even at 500° C. reaction temperature. The long term (100 h) performance tests over 5Ni/Ce0.5Zr0.33Ca0.17O2 catalyst for CO2 reforming of CH4 were extremely successful [28].
This set of results was thought to be useful for biogas which is usually saturated with water. Consequently, the three catalysts i.e., 5Ni/Ce0.5Zr0.33M0.17O2 (where M=Ca, La, Y) were tested for CO2 reforming of CH4 using a methane-rich mixture, (CH4/CO2=1.25) at 800° C. operating temperature. From the obtained results it was noted that the 800° C. reaction temperature was not sufficient to enable the CO2 reforming process of methane-rich mixtures on account of severe catalyst coking. In order to tackle the problem of catalyst coking (deactivation) at 800° C., the temperature was raised to 900° C., which resolved the problem of catalyst deactivation [28]. In order to simulate real biogas, 100 ppmv H2S was introduced at this stage. The activity results (800° C. and 900° C. operating temperature) with the above feed revealed that the above catalysts were not suitable for the above application due to the affinity of the ceria-based supports Ce0.5Zr0.33M0.17O2 (where M=Ca, La, Y) to the sulphur compounds (H2S) as shown in FIG. 1.
It is well known in the literature that H2S reacts rapidly with CeO2 forming surface oxysulfide (CeOS) and surface sulfide (CeS2) species.